static electricity and chemical safety...even if bulk/cone discharges are present, no static...

©2019 Stonehouse Process Safety. All rights reserved.

Static Electricity and Chemical Safety

Vahid Ebadat, Ph.D., M.Inst.P., MIET, C.Phys.

Stonehouse Process Safety, Inc.Princeton, New JerseyCall: +1 609 455 0001

Email: [email protected]: www.stonehousesafety.com

Presentation at2019 Spring Meeting

Society for Chemical Hazard CommunicationLas Vegas, NA

March 20th, 2019


Static Electricity and Chemical Safety

Presentation Outline

• Introduction

• A Systematic Approach to Electrostatic Hazard Assessment

• Control of Electrostatic Hazards

• Discussions


Fire Triangle

• Fuel - A gas, liquid vapor, mist, or solid material capable of being oxidized

• Oxidant - A material that may cause or enhance the combustion of other materials, usually oxygen in air

• Ignition source - An energy source capable of initiating a combustion reaction

©2019 Stonehouse Process Safety. All rights reserved. Page 5

Hazard Management

Ensuring Safety Through Prevention & Protection

• Elimination of fuel

• Elimination of oxidant

• Elimination of ignition sources

• Explosion protection


NFPA 77, Recommended Practice on Static Electricity (Ref. NFPA, 1 Batterymarch Park, Quincy, MA 02169)

• Purpose - Provide assistance in controlling the hazards associated with the generation, accumulation, and discharge of static electricity

• Contains: o A basic understanding of the nature of static

electricityo Guidelines for identifying and assessing hazards

of static electricityo Techniques for controlling the hazards of static

electricityo Guidelines for controlling static electricity in

selected applications


Electrostatic Hazard Assessment - Systematic Approach

Charge GenerationCharge Generation

Contact

Induction

Corona

Charge Accumulation

Charge Accumulation

Isolated Conductors

Insulating Objects

Personnel

Powders

Liquids

Electrostatic Discharges

Electrostatic Discharges

Spark

Brush

Propagating Brush

Cone

Corona

Minimum Ignition Energy

Minimum Ignition Energy

Smallest static spark energy to

ignite a flammable

atmosphere

Flash Fire / explosion?Flash Fire / explosion?

Ignition occurs if discharge energy

> MIE


Charge Generation - Contact Charging

Picture Ref. Lumen Learning

• Electrostatic charge is usually generated when any two materials make and then break contact, with one becoming negative and the other positive

• Build up of the charge on electrically isolated conductors and/or on insulating materials, can give rise to electrostatic discharges

• Ignition is expected if discharge energy is greater than Minimum Ignition Energy (MIE) of the flammable atmosphere


• Personnelo Walking on insulating flooringo Removing coveralls while wearing

insulating shoes • Powders

o Sievingo Pouringo Auger or screw-feed transfero Grindingo Micronizingo Pneumatic conveying

• Liquidso Liquid transfer in hoses & pipeso Mixing / Agitationo Filtration

• Movable Itemso Metal carts with rubber/plastic wheel

Electrostatic Charge Generation -Examples


Electrostatic Data / Properties

Proper assessment of electrostatic hazards requires certain information on properties of plant and the materials handled/processed, such as:

• Minimum Ignition Energy of flammable atmospheres*• Resistance-to-Ground of conductive (metal) plant and

objects**• Resistance-to-Ground of operators’ footwear and floors**• Volume Resistivity of powders*• Volume Resistivity of hoses*• Conductivity of liquids*• Surface Resistivity of containers and liners*• Electrostatic Chargeability of powders and liquids*/**

* Laboratory Measurement** On site measurement


Typical Minimum Ignition Energy Values

1. Values quoted in this table must not be used for the determination of explosion prevention and/or protection measures. Factors such as test method, composition, particle size, moisture content may affect the results. Test data from a representative sample from your own process must be used

Flammable Atmosphere Minimum Ignition Energy (mJ)

Gas / Vapor (ref. NFPA 77, 1 Batterymarch Park,

Quincy, MA 02169)

Carbon DisulfideHydrogenAcetyleneMethanolAcetoneMethane

Ethyl AcetatePropane

0.0090.0160.0170.140.190.210.230.24

Dust Cloud1

(ref. various sources)

PVCZinc

Wheat FlourPolyethylene

SugarSulphur

AluminumEpoxy ResinZirconium

1,500200503030151095


Controlling Electrostatic Hazards - Metal Plant

• Accumulation of static charges can be prevented by grounding isolated conductors

• Grounding - The process of bonding one or more conductive objects to the ground, so that all objects are at zero (0) electrical potential (voltage); also referred to as earthing

• A resistance <106 ohms is generally adequate• All metal system, resistance in continuous

ground paths typically is <10 ohms• Effective bonding & grounding requires:

o Identification of conductive equipment and objects within a process

o Periodic inspection and testing of bonding and grounding systems


Controlling Electrostatic Hazards - Metal Containers

• Ensure grounding at all times during filling and emptying operations

• Apply grounding clamp prior to commencing filling and emptying operations

• Use grounding clamp with hardened steel points that will penetrate through paint, corrosion, and material buildup


• Uncoated fiberboard (including paper & wood) is static dissipative

• Ensure grounding through the metal chime at all times during filling and emptying operations

• Apply grounding clamp prior to commencing filling and emptying operations

Controlling Electrostatic Hazards - Fiberboard Containers


• Insulating Hoses with Imbedded Metal Spiral Wire

o Ground metal wire at both ends of the hose

o Propagating Brush discharges are still possible from inside surfaces of the hose

o Use “conductive” or “static dissipative” hoses

Controlling Electrostatic Hazard


Controlling Electrostatic Hazards - Personnel

• Human body is an electrical conductor and can accumulate static charge if insolated from ground

• During normal activity, the voltage (potential) on the human body can typically reach 10kV to 15kV. At a capacitance of about 200pF the accumulated energy available for a spark can reach 10mJ to about 30mJ

• A person insulated from ground can accumulate a significant charge by:o Walking on an insulating surfaceo Manual pouring of powders and liquids from one container to another while wearing

insulating footwear or standing on insulating flooringo Touching an already charged object such as a Type A, B, and D FIBCs (supersacks)

during filling and emptying o Brushing against surfaces while wearing insulating footwear

• Electrostatic sparks from human body have been responsible for numerous flash fire and explosion incidents


Controlling Electrostatic Hazards - Personnel

Illustration of static dissipative footwear tester (Source: Neilson-Gail.com

• Static dissipative footwear used together with conductive or static dissipative flooring provides a practical means to control and dissipate static charges from the human body

o Resistance to earth through static dissipative footwear and conductive or static dissipative flooring should be between 106 ohms & 108 ohms

• Grounding wrist straps can also be used but may not be practical where personnel need to move around


Electrostatic Hazards - Insulators

• Examples - plastic hoses, bags, liners, drums

• Surface Resistivity >1011

ohm/square• Grounding of insulating objects

would not typically result in relaxation of charge to ground

• Build up of static charge• Insulating containers prevent

the relaxation of static charge from their contents (powders and liquids) to ground


• Consider conductive or static dissipative materials with a Surface Resistivity <1011 ohm/square

Controlling Electrostatic Hazards - Insulators


Controlling Electrostatic Hazards - Plastic Liners

• Depending on breakdown voltage of the liner and product charge density there is a possibility of “propagating brush” discharges

• Use antistatic or conductive liners with Surface Resistivity <1011

ohm/square• In the absence of flammable vapor

atmospheres insulating liners may be used if breakdown voltage of liner is <4kV


• Electrostatic charge generation can arise in various liquid handling operations such as filling, sampling, filtration and mixing

• Electrostatic charge can accumulate on:o Low conductivity liquids

o Liquids in non-conductive containers and vessels

o Liquid mist regardless of liquid conductivity

Controlling Electrostatic Hazards - Liquids



• Use Electrically Grounded Conductive (Metal) Planto All plant items such as pipes, vessels, containers etc.

should be electrically conductive and/or static dissipative and grounded

• Increase Liquid Conductivityo Where a conductive and a nonconductive liquid are to be

blended, add conductive liquid first o Adding a conductivity-enhancing agent (antistatic

additive Antistatic additives normally are added in parts-per million

concentrations



• Control of Liquid Entry to Vesselo Design inlet to minimize jetting of

highly charged product to the surface and to minimize the disturbance of the vessel contents

o Avoid splash filling through bottom filling or by using an inlet dip pipe extending close to the tank bottom

o Where the process requires top filling use a fill pipe directed toward the inner wall of the tank


Insulating Containers for Liquids

• How about these items?


• Bulking of highly charged insulating powder in containers causes a partial surface discharge over the top of powder pile that appears as a luminous, branched channel flashing radially from the wall toward the center of the pile - “Bulk”/”Cone” Discharge

• Discharge energy depends on powder “Volume Resistivity”, “Electrostatic Chargeability”, “particle size”, and “vessel dimensions”

• Bulk/Cone discharges have been attributed to explosions of dusts having MIEs less than 20mJ

Controlling Static Hazards - Powders


Controlling Electrostatic Hazards - Powders• Assessment of ignition hazards caused by Bulk/Cone

discharges:

o Powder Volume Resistivity <109 ohm.m

No electrostatic charge accumulation and hence no “Bulk” discharge if powder is handled in grounded conductive plant

o Powder Volume Resistivity >109 ohm.m andMinimum Ignition Energy >20mJ

Even if Bulk/Cone discharges are present, no static ignition hazard in grounded conductive plant

o Volume resistivity > 109 ohm.m and Minimum Ignition Energy <20mJ

For sufficiently high electrostatic charging (high chargeability) conditions possibility of Bulk/Cone discharges cannot be ruled out. Consider:

Inert gas blanketing, or

Explosion protection, or

Reduction of electrostatic charges (ionization)


Controlling Electrostatic Hazards

• Effect of Humidity on Charge Accumulation

o Water vapor - if present in air - absorbs onto surfaces and forms a slightly conducting surface layer

o Factors affecting the extent of water absorption onto the surface include: Chemical properties of the surface, and

Quantity of moisture content (humidity) in air

o Maintaining the relative humidity level higher than about 65% can reduce electrostatic charge accumulation


Controlling Electrostatic Hazards

• Effect of Humidity on Charge Accumulationo Electrostatic Voltages Resulting from (Contact) Triboelectric Charging at

Two Levels of Relative Humidity (RH) (Ref: NFPA 77, 1 Batterymarch Park, Quincy, MA 02169)

SituationElectrostatic Voltages (kV)

RH 10% - 20% RH 65% - 90%

Walking across a carpet 35 1.5

Walking across a vinyl floor 12 0.25

Working at a bench 6 0.1

Vinyl envelopes for work instructions 7 0.6

Poly bag picked up from bench 20 1.2

Work chair padded with polyurethane foam 18 1.5


IEC 61340-4-4, Edition 3.0, 2018-01

Standard Test Methods for Specific Applications –Electrostatic Classification of Flexible Intermediate Bulk Containers (FIBCs)


IEC 61340-4-4, Edition 3.0, 2018-01• Type A - Constructed from insulating fabric or plastic sheet

with no provision for controlling any type of static discharge o Must not normally be used in presence of flammable atmosphere

with MIE ≤1,000mJ

• Type B - Constructed from insulating fabric or plastic sheet designed to prevent “Sparks” and “Propagating Brush Discharges”

o Safe for use in dust environments with ignition energies greater than 3mJ. There must be no flammable vapors/gases present

• Type C - Constructed from fully inter-connected conductive threads

o Relies on grounding to prevent electrostatic hazards (shall have a resistance to groundable point of <1.0x108 Ω)

o Safe for use in the presence of flammable vapors, gases, or dusts with MIE ≤3mJ

• Type D - Constructed from fabrics with special static protective threads and/or properties

o Controls discharge incendivity without requiring earthingo Safe for use where flammable vapors, gases, or dusts with MIE

≤3mJ are present


Use of Different Types of FIBC

Bulk Product in FIBC Surroundings

MIE of DustNon Flammable

AtmosphereDust Zones 21 – 22

1000mJ > MIE > 3mJ

Gas Zones 1 – 2 or Dust Zones 21 – 22

MIE ≤ 3mJ

MIE > 1,000mJ A, B, C, D B, C, D C, D

1000mJ > MIE > 3mJ B, C, D B, C, D C, D

MIE ≤ 3mJ C, D C, D C, D

o Additional precautions are usually necessary when a flammable gas or vapor atmosphere is present inside the FIBC, e.g. in case of solvent wet powders

o Non-flammable atmosphere includes dusts having a MIE >1000mJo Use of Type D FIBCs shall be limited to atmospheres with MIE ≥0.14mJ


Explosion Protection

Ignition Source

Ensuring Safety Through Prevention & Protection

• Explosion Preventiono Elimination of Fuelo Elimination of Oxidanto Elimination of Ignition Sources

• Explosion Protectiono Protection measures must be considered when preventative measures on

their own may not ensure acceptable level of safetyo Explosion protection measures include:

Explosion venting to a safe place (NFPA 68) Explosion suppression by injecting a suppressant (NFPA 69) Containment by explosion resistant construction (Design based on ASME Boiler

and Pressure Vessel Code, Section VIII, Division I)


Thank You

We partner with our clients in the process industries to help them identify, assess, prevent, and control fire, explosion, and accidental loss of containment hazards in their operations.

We would like to hear from you. Please contact us!

Vahid Ebadat, Ph.D.Stonehouse Process Safety, Inc.Princeton, New JerseyCall: 609 455 0001Email: [email protected]: www.stonehousesafety.com

static electricity and chemical safety...even if bulk/cone discharges are present, no static...

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